Antibodies are produced by the immune system to fight infections. So, if someone has antibodies to the SARS-CoV-2 virus (that causes COVID-19), it tells us that they’ve been infected by the virus at some point, but it doesn’t tell us exactly when – and here’s why that’s important:
If a person gets infected with SARS-CoV-2, their body usually starts producing antibodies within one to three weeks – but the presence of antibodies doesn’t necessarily mean they’re not contagious, because this period can overlap with the contagious period for COVID-19. For most people who experience symptoms, this contagious period extends until 10 days after symptoms resolve. People who test positive for the virus – but have no symptoms (asymptomatic carriers) are also not likely to be contagious after 10 days—but there are documented exceptions.
So, in other words, you can test positive for antibodies while you’re still contagious. In fact, some people only discover they’re infected with the coronavirus after a positive antibody test triggers a viral test (the COVID-19 nasal swab) that confirms they have an active infection.
While a positive COVID-19 antibody test could mean you are currently infected with SARS-CoV-2, it’s more likely an indication that you were infected sometime in the past and are no longer contagious. People who test positive for antibodies and also have symptoms of illness should consult their doctor to see if any further testing should be performed (such as a viral test).
So, when you encounter someone who says they had a “positive antibody” test, let their symptoms (or lack of them) guide how you interact with them. If they had a positive antibody test but don’t have any symptoms of COVID-19, then it’s unlikely you might catch COVID-19 from them. You can follow general precautions, such as social distancing and mask wearing, when you’re around them.
But if they had a positive antibody test and they are feverish, coughing, or have other symptoms of illness, then you should avoid close contact with them if at all possible. If you must interact with them, be sure you wear a mask (and have them wear one, too) and perform good hand hygiene after every encounter.
And keep in mind that there’s still a lot we don’t know when it comes to our immune system’s response to the virus that causes COVID-19. We don’t know for certain exactly how our antibodies behave, how long our antibodies remain effective at neutralizing the virus, or why some people don’t produce antibodies to this virus. So, stay informed with the latest news on the virus to make sure you’re doing all you can to stay healthy.